US smoking behavior is undergoing dramatic change: According to BRFSS, 25% of all adult smokers are now intermittent smokers (ITS) who do not smoke daily - an increase of 40% over 5 years (1996-2001). Understanding ITS is important for several reasons: ITS smoking patterns challenge current models of smoking behavior, ITS smokers incur substantial health risk, some ITS smokers have trouble quitting, and we lack knowledge of how to intervene in ITS. Estimates suggest that ITS is growing and will continue to grow, becoming a prominent smoking pattern that theory and interventions must be able to address. Although epidemiological surveys have characterized the demographics of ITS, the field lacks substantive information about ITS: why they smoke;how they smoke, and when they smoke. It is essential to fill these gaps in knowledge to more fully understand factors that control smoking behavior, to refine theories of nicotine dependence and smoking, and to develop more precisely directed interventions that will assist this growing segment of the smoking population. To fill these gaps, we propose an intensive case-control study contrasting 300 ITS and 300 daily smokers (DS) recruited from the community via random-digit dialing. We hypothesize that stimulus control - the association of smoking to particular stimulus contexts - is a key distinguishing feature of ITS compared to DS. We propose to study stimulus control in ITS and DS in two ways: (1) In a naturalistic field study, subjects will track the stimuli associated with smoking and non-smoking occasions for two weeks using electronic diaries and Ecological Momentary Assessment methods. (2) In a randomized laboratory experiment using cue-exposure methods, we will assess craving responses to a panel of cues (e.g., negative affect, positive affect, alcohol). Additional analyses will compare groups on smoking history and dependence and use baseline data to predict prospectively ITS transitions to daily smoking and to abstinence over a follow-up of up to 4 years. Thus, our study focuses on individual differences in stimulus control of smoking as an important process that may be associated with patterns of regular and addictive smoking. These studies have the potential to shed some much needed light on ITS as an important emerging smoking pattern, inform theories of nicotine dependence, and suggest directions for intervention both with ITS and with regular daily smokers.